An “outside the box” sequence

I found a small letters sequences on Stack Exchange, but some elements are missing :

T H B C D L PP WH ?? M CU ?


Question

What are the missing elements ?

Hints

Hint 1 :

Hint 2 :

Every ? is a letter.

Hint 3 :

Think outside the box.

T H B C D L PP WH AI M CU F

This is from

the first letters of the links across the top of the site footer

• That was fast. Veeery fast. You got it ! – The random guy Aug 20 '15 at 12:15
• @Somo145 Really Awesome !!! – kanchirk Aug 20 '15 at 12:15
• @Therandomguy You have to be quick here! – Somo145 Aug 20 '15 at 12:18
• Yeah, but i also need to increase difficulty on my questions D: – The random guy Aug 20 '15 at 12:19
• @CodeNewbie - I had the right thought (outside the box, so I looked round the edges of the website page) but I only looked at the header, I didn't think of the footer as it was off-screen. – AndyT Aug 20 '15 at 15:12